Femtosecond Laser Excitation in Argon–Nitrogen Mixtures

Abstract

Argon gas is demonstrated to enhance femtosecond laser electronic excitation tagging at atmospheric pressure and temperature for unseeded velocimetry applications, primarily through two to four orders of magnitude increase of excited species that may radiate through nitrogen’s second positive system at early timescales of interest. The first positive system continues to play an important role in maintaining this emission at longer delays. A detailed kinetic model is implemented to explain this observed behavior in nitrogen and argon mixtures. Dominant processes governing the creation of and include a slower decay of electron temperature through increased ionization processes, reduced nitrogen quenching of electrons, nitrogen atom creation and recombination, the formation and dissociation of , and a number of argon–nitrogen direct and indirect excitation pathways. The production of ions through charge-transfer reactions and affect excited C- and B-state nitrogen population delays at later times (). The pooling reactions play minor roles in the formation of and at timescales useful for measurements in the femtosecond laser electronic excitation tagging argon plasma chemistry. In mixtures where nitrogen is dominant, metastable argon species are less instrumental in direct nitrogen excitation transfer, , than in facilitating further reactions through maintaining a higher electron temperature, whereas this excitation transfer begins to play a larger role as the percentage of argon is further increased. The model yields results that agree with sub-100 torr argon–nitrogen discharge experiments and theoretical results derived from other studies. It is concluded that not one single process can be credited for the enhancement, but a combination of ionization and heating produces the increased emission observed in argon mixtures.